1. Technical Field
The present invention relates to a method and system for isolating and reconnecting a dual port bus from a primary bus.
2. Description of Related Art
It is well known to interconnect peripherals in a computer system by means of a bus. For example, a host processor may be connected to a number of disk drive peripherals by a bus, such as a standard SCSI (small computer system interface) bus. An advantage of using a SCSI bus is that there are many standard, commercially available peripherals, such as SCSI disk drive peripherals, which can be connected to the bus.
A failure of any peripheral on a bus has the potential to bring down the entire bus and thereby shutting down every other peripheral and/or component on the bus. In a worst case scenario, a catastrophic system failure can be precipitated by a failure of any single peripheral on the bus. However, even if the other peripherals on a bus remain operable following the failure one peripheral, the bus performance may be compromised and communication between a host and the peripherals impaired.
A dual port enclosure has the advantage of accessing the main bus via a second port if the primary or first port causes errors on the main bus. A technician can perform a test and recovery procedure intended to identify the likely source of the bus errors and then attempt to recovery the main bus by manually isolating the faulty device from the main bus. If the faulty device is attached to an external bus within an enclosure, the entire enclosure and all of its functionality must be isolated from the main bus. If the enclosure is dual ported, the technician may be able to restore the functionality of the enclosure to the main bus by logically connecting the second port to the main bus and then recheck the main bus for errors. However, unless a technician is available, the advantages of the dual port enclosure are lost.
It would be advantageous to provide an automated means for isolating an expansion bus from a main bus. It would be further advantageous to improve techniques for controlling logical isolation and logical connection of a dual port bus.
The present invention relates to a system and method for providing more complete control of a dual port bus. There are four primary components: two enclosure monitors; and two SCSI bus expanders with isolation circuitry. The sub-system is configured such that an enclosure monitor and an expander are both connected to an external port, but not necessarily the same external port. The internal bus then connects the two expanders, as well as all of the internal SCSI devices (e.g. hard drives, CD-ROMs, tape drives). The enclosure monitors can communicate with various host devices over the external SCSI buses. These host devices can instruct the enclosure monitor to either connect or isolate the internal SCSI bus; thereby the peripherals attached to it. This is accomplished through a set of independent control signals that run between the monitor and the expander. There are three different methods of control. The first is independent, paired control between enclosure monitor/bus expander pairs. A separate host controls each enclosure monitor/bus expander pair. The second involves the enclosure monitor/bus expander pairs having cross over control between the ports. There, a single host sends requests to both pairs, but using the opposite port as the port to be isolated. The recipient enclosure monitor controls the opposite port from its connection port. The third method utilizes two dependent enclosure monitor/bus expander pairs. The two enclosure monitors are connected through an interconnection bus and can communicated isolation states as well as pass on isolation requests.